Effect of enzymic methylation of heterogeneous ribonucleoprotein particle A1 on its nucleic-acid binding and controlled proteolysis

Recombinant unmethylated heterogeneous nuclear ribonucleoprotein particle (hnRNP) protein A1 was enzymatically methylated by nuclear protein/histone protein methylase I [Rajpurohit, Lee, Park, Paik and Kim (1994) J. Biol. Chem. 269, 1057-1082] and the effect of methylation on several physiocochemical properties was studied. The relative binding-affinity of methylated and unmethylated protein A1 to nucleic acid was quite different. This was observed by the elution behaviour of the protein A1 on a single-stranded DNA/cellulose column; the concentration of NaCl required to release the bound protein A1 was 0.59 M for the methylated and 0.63 M for the unmethylated, respectively. Employing isoelectrofocusing, pI values of the methylated and unmethylated proteins were found to be 9.41 and 9.48, respectively. Maximum fluorescence quenching of protein A1 in the presence of coliphage MS2-RNA was found to be 40% with methylated and 45% with unmethylated. When both species of protein A1 were subjected to controlled trypsin digestion, t1/2 of the methylated protein was 1.31 min and the unmethylated, 1.63 min. The difference in their t1/2 values was much greater in the presence of MS2-RNA; 2.4 min for the former and 4.3 min for the latter, indicating that the methylated species was less stabilized by the RNA than the unmethylated. All of the above results consistently suggested that the binding-property of hnRNP protein A1 to single-stranded nucleic acid was significantly reduced subsequent to its arginine-methylation. The biological significance of this observation is discussed.